Tunable parametric amplifier with signal frequency higher or lower than pump frequency



April 5, 1966 B. J. LEON 3,244,992

TUNABLE PARAMETRIC AMPLIFIER WITH S IGNAL FREQUENCY HIGHER OR LOWER THANPUMP FREQUENCY April 5, 1966 B. J. LEON 3,244,992

TUNABLE PARAMETRIC AMPLIFIER WITH SIGNAL FREQUENCY HIGHER OR LOWER THANPUMP FREQUENCY Original Filed Jan. 16, 1962 5 Sheets-Sheet 2 L Era-5./y/

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April 5, 1966 B. J. LEON TUNABLE PARAMETRIC AMPLIFIER WITH SIGNALFREQUENCY HIGHER OR LOWER THAN PUMP FREQUENCY Original Filed Jan. 16,1962 3 Sheets-Sheet 3 United States Patent Office 3,244,992 PatentedApr. 5, 1966 3,244,992 TUNABLE PARAMETRI@ AMPLEFIER WlTH SGNAL FREQUENCYHlGHER 0R LOWER THAN PUMP FREQUENCY Benjamin J. Leon, West Lafayette,Ind., assignor to Hughes Aircraft Company, Culver City, Calif., acorporation of Delaware Continuation of application Ser. No. 166,593,Jan. 16, 1962. This application May 28, 1964, Ser. No. 373,900 8 Claims.(Cl. S30- 4.9)

This invention relates to parametric amplifiers and particularly to atunable single stage linear parametric amplifier.

This application is a continuation of a copending application ofBenjamin I. Leon, Serial Number 166,503, filed January 16, 1962, and nowabandoned, entitled,

Tunable Parametric Amplifier.

Parametric amplifiers which include ia non-linear reactive element, anidler circuit and a source of pumping energy provide low noiseamplification. Conventional parametric amplifiers operate at a selectedfrequency in response to two tuned circuits, one being tuned at a signalfrequency and the other being tuned at an idler frequency to developamplification at a frequency that is the difference o-f the pumpingfrequency and the idler frequency. Thus, these conventional amplifiersare only tunable by varying both the pumping frequency and other circuitparameters. Another disadvantage of conventional parametric ampliiiersis the large numlber of circuit elements required for providing sharpskirts for the tuned circuits.

It is therefore an object of this invention to provide a tunablenegative resistance parametric amplifier that is tuned by varying thepump frequency with all circuit elements within the amplifier havingfixed values.

It is another object of this invention to provide a parametric ampliercircuit which may tbe tuned to amplify a source of signals over a verywide frequency range by varying only the pumping frequency.

It is a further object o-f this invention to provide a parametricamplifier circuit that provides amplification of the input atfrequencies equal to both the sum and the dierence of the pump and idlersignal frequencies.

it is a still further object of this invention to provide a simplifiedparametric amplifier lcircuit that is resonant at only the idlerIfrequency.

Briefly, in accordance with this invention, a simplified negativeresistance parametric amplifier is provided that is tunable over a verywide frequency range by varying only the pumping frequency. A source ofinput signals and a source of pumping energy apply voltages to anonlinear capacitan-ce element which in turn is coupled to a resonantidler tank circuit tuned to the idler frequency. The fixed parameterswhich form the idler tank circuit and other tuned isolating elements ofthe circuit are such that the admittance has a sharp resonance at theidler frequency and that the pumping frequency is larger than the idlerfrequency. The circuit provides amplification at a frequency equal tothe difference between the pump and the idler frequencies. `In anotherarrangement in accordance with this invention, which provides the idlertank circuit with a very sharp resonant frequency, amplification canalso be obtained at a frequency equal to the sum of the pump and theidler frequencies.

The novel features of -this invention, as well as the invention and itsmethod of operation, will best be understood from the following detaileddescription taken in connection with the accompanying drawings, inwhich:

FIG. 1 is a schematic circuit diagram of one arrangement of a tunableparametric amplifier circuit in accordance with this invention;

FIG. 2 is a schematic circuit diagram of an equivalent circuit of thenon-linear capacitance element of FIG. 1 as it appears to the inputsignal frequency voltage.

FIG. 3 is a -schematic circuit diagram of the equivalent circuit of thepumped diode of FIG. 1;

FIG. 4 is a schematic cir-cuit diagram of another arrangement of atunable parametric amplifier circuit in accordance with this inventionwhich provides amplification at both the sum and the difference of thepump and idler frequencies;

FIG. 5 is a graph of impedance versus frequency for showing the Wideband operation of lthe circuits of FiGS. 1 and 4;

FIG. 6 is a graph of negative conductance versus input signal frequencyshowing the negative conductance developed rby t-he circuit of FIG. 1 ata selected pump frequency;

FIG. 7 is a graph of the logarithm of negative resistance versus thelogarithm of signal frequency showing the points of maximum negativeseries resistance developed in the circuit of fFIG. 1 by tuning over awide range of signal frequencies;

FiG. 8 is a graph of negative conductance versus signal frequencyshowing part of the negative conductance developed by the circuit ofFIG. 4 at a selected pump frequency;

FIG. 9 is a graph of negative conductance versus signal frequencyshowing another part of .the negative conductance developed by thecircuit of FIG. 4 at the same selected pump frequency as that of thegraph of FIG. 8; and

FIG. 10 is a graph of the logarithm of negative resistance versus thelogarithm of signal frequency showing the points of maximum negativeresistance developed in the circuit of FIG. 4 over a wide range ofsignal frequencies.

Referring first to the parametric amplifier circuit of FIG. 1, an inputsignal source 10 may have one end coupled to a level of referencepotential such as ground and to an input terminal 1l and the other endcoupled to a lead 12 for applying an input signal ES through a sourceresistor 16 having ya value RS to an input terminal 17. The source 10may tbe a signal generator or any source of signals to be amplified andmay provide the input signal ES at different frequencies. The inputterminal 17 is coupled -to a lead 18 and also to a terminal 2G, with aload resistor 24 having a value RL being coupled between the terminal4Zti and a terminal 26 which in turn is coupled to ground. The lead 13is also coupled to a non-linear (voltage variable) capacitance element28 which may be a conventional voltage variable nonlinear capacitancediode 27 such as a varactor. The element 28, as shown 'by the equivalentcircuit of the diode 27 in FIG. 2 at the frequency of the input signalES, will include a negative conductance 29 having a value -G which listhe negative conductance presented by the .varia-ble capacitive elementat the frequency of the in- Aput signal ES.

Also, the element 28 includes a fixed capacitance 30 having a value ofC0 coupled in parallel with the negative conductance 29. To furtherenplain the characteristics of the element 28, the equivalent circuit ofthe pumped diode of FIG. 3 shows a voltage variable capacitor 31 havinga value C1 cos wp! where wp is the radian frequency of a pumping signal.

A source 32. of selectively variable pumping signals EP at the radianfrequency wp is provided, with one end coupled to ground, to apply thepumping signals through a pump resistor 36 having a value lRp to thelead 1S and to the non-linear capacitance element 23 to develop aneffective sinusoidal capacitance variation C1 cos opt as seen 3 in FIG.3. The source 32 may be any conventional source of variable frequencysignals such as a variable frequency oscillator as is well known in tbeart. The pumping source 32 and the capacitance element 28 together formaV time variant capacitor 41 having the characteristics of FIG. 3. Thetime variant capacitor 41 may Within lthe principles of this linventionalso be a motor driven butteriiy condenser, for example, coupled in thecircuit atA the same location as the non-linear element 23. An idlercircuit 40 which is a pa-rallel resonant tank circuit tuned' at theidler frequency includes a resistor 42, a capacitor 44 and an inductor46 coupled in parallel between ground and a lead 48 which in turn iscoupled to the capacitance element 28. To provide biasing to the diode27, a battery 45 and by-pass capacitor 47 are coupled in parallel1oetween one end of the inductor 46 and the ground terminal 11. Theidler circuit 4) is tuned to be resonant at an idler radian frequency w1equal to (wy-ws) where wp is the radian frequency of the signal EPsupplied by the pump source 32 and ws -is the radian frequency of thesignal ES applied to the circuit from the input source 16.

Thus, the frequency of the pumping source 32 is varied to provide tuningwhen the frequency of the source 16 of input signals varies, as theidler circuit 40 is maintained tuned at the frequency (wP-ws). As willbe discussed subsequently, the circuit of FIG. 1 provides amplificationat the input signal frequency across the load resistor 24. The circuitprovides resonance which is the equivalent of parallel resonance at theidler frequency wi through the capacitance element 28, by a current pathincluding the element 28, the inductor 46 and the load resistor 24. Itis to be noted that in the circuit of FIG. 1, only the oneresonantcircuit `44B tuned to the idler frequency is required foramplification.

The parametric amplifier circuit ofV FIG. 4 includes a source 52 ofinput signals ES having one en d coupled to ground and to an inputterminal 53 and having the other end coupled to -a lead 54 which in turnis coupled through a source resistor 58 providing a resistance of RS toan input terminal 55. A lead 56 is coupled from the terminal 55 via aresistor 88 to one end of a non-linear capacitance element 62 which mayinclude a voltage variable nonlinear capacitive diode 63 such as avaractor. When pumped from a source of pump signals S2, the non-linearcapacitance element 62 -appears as a time variant capacitance thatdevelops the sinusoidal capacitance variation C1 cos wpt as shown in FG.3. An `equivalent negative con- Y ductance -G and fixed capacitance C0similar to therespective parallel coupled negative conductance 29 Iandcapacitor Sil 4of FiG. 2 is .also presented .to the input signal ES bythe capacitance element 62. A first resonant cir'- cuit 66 tuned at theidler frequency for provid-ing a short circuit to ground to isolate theidler circuit from the input signal source 52 includes an inductor 68anda capacitor 76 coupled in series between the lead 56 and -a groundlead 74. A tuning capacitor 78 is provided between the leads 56 and '74.The source 82 of pumping signals EP has one end coupled to the groundlead 74 and the other end coupled through a pumping source resistor `86having a value RP to .the lead 56. The resistor 86 provides isolaltionof the pumping source 82 from the element 62. VThe pumping source 82 iscontrollable to provide a variable frequency for tuning the frequency ofamplification of .the signal ES. The input signal source 52 may be anysource of signals to beamp'lified and which may change in frequency.Also, the pumping source 82 may be any signal generator of controllablefrequency such as a signal generator utilizing voltage variablenon-linear reactive elements. The pumping source 82 and the non-linearcapacitance element 62 together form a time variant capacitor `83 havingthe characteristics of FIG. 3 as discussed relative to FIG. 1.

To provide series resonance with the capacitance of the capacitanceelement 62 at the idler frequency, an inductor is coupled between a lead91 and a lead 92 whichv need not be small at any other frequencies.

in turn is coupled to the other end of the non-linear diode 63. Abattery 93 and by-pass capacior 95 are coupled in parallel between theleads 74 and 91 to provide biasing lto the diode 63. The idler currentsWhich .pass in series through the element 62 and the inductor 90 thuspass through the resonant circuit -66 or the capacitor '78. In order tokeep the idler energy from the load circuit, a. capacitor 96 andinductor 98 of a resonant circuit 97 are coupled in parallel between thelead 91 and a lead 99 which in turn is coupled through a winding 106 ofa transformer 168m the lead 92. The parallel resonant circuit 957 istuned at the idler frequency to present a high impedance to signals atthe idler frequency but is nonresonant or presents a low impedance tosignals at the input signal frequency. The transformer 10S `is providedwith a second winding 112 coupled through terminals 114 and 116 to aload resistor 118 which may have a value RL. The transformer 16S, whichmay have a 1:1 ratio, allows the terminal 116 to be grounded.

Referring back to FIG. l for further explaining the operation of :theparametric amplifier thereof, the circuit provides amplification at asignal frequency ws equal to (wp-wi) with a minimum of circuit elements.The admittance Y(w) seen by the variable part of the capacitance of FG.'3 has been determined for the circuit of FIG. 1 having the followingvalues, for example:

Resistor 16, Rs:0.5 o-h-m Resistor 24, RL:0.5 ohm Resistor 36,RP=relatively large Capacitor 31B, C0:4 farads YResistor 42:25() ohmsCapacitor 44:16 farads Inductor 46:5 henries Pump source adjusted sothat C1, the amplitude of the variable capacitance of the capacitor 31is 2 farads.

The admittance Y( w) is as follows for these values:

where w is the radian frequency such as the idler frequency, pumpingfrequency or inputsignal frequency and ,i is the conventionaldesignation for imaginary numbers. Equation 1 shows that resonance ispresent, i.e., Y(w) is very small, when the idler frequency w1 isapproximately equal to 0.10 radian/second. A necessary condition for thecircuit in accordance with this invention is that the admittance must besmall at the idler frequency wi and The 4]'w 'term of Equation 1 insuresa broad response at other interesting frequencies. Another requirementof the circuit of FIG. 1 is that the idler frequency as determined bythe resonant frequency of the circuit 40 be less than the pumpingfrequency.

As shown by a curve 120 in FIG. S the impedance VThe curve 129 showsthat a requirement of the circuit in accordance with this invention thatonly one resonant Ifrequency is utilized, is satisfied .tol provide thenegative resistance amplifier. v

i The negative conductance -G developed by the non linear capacitanceelement 28 rises above zero to a maximum peak 122 at a signal frequencyws of approximately 0.899 radian/second for a pumping frequency wp of0.1 radian per second as shown by a curve 124 of FIG. 6. Operation ofthe circuit of FIG. 1 was simulated on a computer with the values givenabove to provide the curve 124. Curves for other pumping frequencieswere determined, and with an increasing pumping wp, the maximum negativeconductance peak 122 moves to the right for larger signal frequenciesws. Thus, the circuit may be tuned to provide a negative conductanceregion and amplification at dierent signal frequencies because ws:(wp-w1) and wp may be Varied.

The maximum negative conductance peaks such as 122 for various values ofinput signal frequency are plotted in FIG. 7 as a curve 128 byexpressing the negative conductance as an equivalent negative resistance-R in series with a capacitance. The curve 12S shows that a negativeresistance and amplification is developed over an input signal frequencyrange of approximately 100:1. It is to be noted that the negativeresistance and amplification decreases at higher signal frequencies butthis may be taken into consideration in designing a system utilizing theparametric amplifier in accordance with the invention.

Referring now to FIG. 4 the operation of the parametric amplifierthereof in accordance with the invention will be explained in furtherdetail. The circuit of FIG. 4 has elements selected so that thenecessary cond-ition in accordance with this invention that w1 is lessthan wp is satisfied and the conventional condition that the capacitanceelement 62 has the property that the capacitance is always positive issatisfied. The admittance Y(w) has been determined for the followingelement values:

Resistor S8, Rs:0.2 ohm Resistor 86, RP:a relatively large valueResistor 88:0.004 ohm Resistor 118 RL:0.31 ohm Inductor 68:1 henryInductor 90:25 Ihenries Inductor 98:6.3 henries Capacitor 70:100 faradsCapacitor 78:40() farads Capacitor 96:16 farads Capacitor 30, (as seenin FIG. 2), C0:4 farads Pump amplitude adjusted so that the variablecapacitance is 2 cos (wpt).

Utilizing these element values, the admittance as a function of w is:

(w (jmwaaooieqawnos where w may be any radian frequency in the circuit.

The bracketed factor in the numerator of Equation 2 provides thenecessary resonance near w1:0.1. The first factor of the numerator ofEquation 2 insures a broad band response at the desired input signalfrequencies.

As may be seen in FIG. 5, the impedance IYI is very small at allfrequencies except at the idler frequency wi which is the only resonancerequired in accordance with this invention. rIlhe circuit is thusbroadband at t-he required frequencies. The circuit of FIG. 4 has beenfound to provide amplification at not only the difference frequency wP-wbut also at the sum frequency wP-i-w. For a predetermined wi thisamplification at the sum frequency of (wP-I-wi) results when the idlerresonance of the circuit 97 is relatively sharp by proper selection ofthe elements thereof. Amplification at the sum frequency can be obtainedonly if the higher frequencies, this includes (Zwpw), (3wpiw), etc., areallowed by the element values. These allowed frequencies are permittedin the circuit of FIG. 4 because of the broad band arrangement shown byFIG. 5. The exact conditions necessary for providing amplification atthe sum frequencies are not completely understood at this time.

The operation of the circuit of FIG. 4 has been simulated on a computerto provide negative conductance regions as shown in FIGS. 8 and 9 withthe previously given examples of element values. At a pumping frequencywP of 1.0, a curve of FIG. 8 shows a negative conductance region withthe input signal ws equal to .8996 radian per second at a peak 142. Atthe same pumping frequency a curve 148 of FIG. 9 shows a negativeconductance peak 149 at an input signal frequency ws of 1.1004 radiansper second. Thus, Ithe circuit of FIG. 4 develops a negative conductanceat a frequency (wp-wi) and (wp-i-wi) so that amplification of the signalEs occurs at both frequencies.

The circuit of FIG. 4 is tunable over -a wide frequency range withnegative resistance to provide amplification as shown by a curve 150 ofFIG. 10 which illustrates the input signal frequency ws when equal tothe difference frequency (wp-wi) versus negative resistance -R. A curve152 shows the input signal frequency ws when equal to the sum frequency(wP-l-wi) as a function of negative resistance -R. As discussed above,tuning is performed by varying the pump frequency wp with the signalfrequency. It is to be noted that the tuning range is limited for sumfrequency amplification as shown by the curve 152.

Thus, there has been described a simplified tunable single-stage, linearparametric amplifier which includes a time variant capacitor and anetwork of fixed resistors, inductors and capacitors. A single sharplytuned idler circuit is all that is required for the amplification inaccordance with the invention. Tuning is performed by varying only thepumping frequency. In one arrangement in accordance with the invention,a sharply tuned idler resonant circuit provides a relatively largeamplification at frequencies of both (wP-i-w) and (wlw-w1) for any tunedvalue of wp.

What is claimed is:

1. A parametric amplifier comprising: first reactance means forproviding a fixed reactance component and a reactance component which isvariable at a pump frequency, means for applying an input signal of aselected frequency to said first reactance means, sec-ond reactancemeans coupled to said first reactance means to provide series resonancewith said fixed reactance component at an idler frequency equal to themagnitude of the difference between said selected frequency and saidpump frequency, a parallel resonant circuit coupled across said secondreactance means and sharply tuned to said idler frequency, and a seriesresonant circuit coupled across said input 'signal yapplying means andtuned to said idler frequency.

2. A parametric amplifier comprising: first reactance means forproviding a fixed reactance component and a reactance component which isvariable at a pump frequency, means for applying a signal to beamplified and of a frequency higher than said pump frequency to saidfirst reactance means, second reactance means coupled to said firstreactance means to provide series resonance `with said fixed reactancecomponent at an idler frequency equal to the difference between saidsignal frequency and said pump frequency, a parallel resonant circuitcoupled across said second reactance means and sharply tuned to saididler frequency, and a series resonant circuit coupled across saidsignal -applying means and tuned to said idler frequency.

3. A parametric amplifier comprising: capacitance means for providing afixed capacitance component and a capacitance component which isvariable at .a pump frequency, means for applying an input signal of aselected frequency to said capacitance means, inductance means coupledto said capacitance means to provide series resonance with said fixedcapacitance component at an spaanse idler frequency equal Vto thedifference between said selected frequency andksaid pump frequency, aparallel resonant circuit coupled across said inductance means andsharply tuned to said idler frequency, and a series resonant circuitcoupled across said input signal applying means and tuned to `said idlerfrequency.

4. A parametric amplifier comprising: capacitance means for providing afixed capacitance component and a capacitance component which isvariable at a pump frequency, means for applying an input signal of afrequency higher than said pump frequency to said capacitance means,inductance means coupled to said capacitance means to provide seriesresonance with said fixed capacitance component at an idler frequencyequal to the difference between .said input signal frequency and saidpump frequency, a parallel resonant circuit coupled across saidinductance means and sharply tuned to said idler frequency, and a seriesresonant circuit coupled across said input signal applying means andtuned to said idler frequency.

5. A parametric amplifier comprising a variable capacitanceV diode,means for applying to said diode a direct bias and an alternating signalwhich varies at a pump frequency to provide in said diode a fixedcapacitance component and a capacitance component which varies at saidpump frequency, source means for applying an input signal of aselectedfrequency to said diode, inductance means coupled to said diodeto provide series resonance with said fixed capacitance component at anidler frequency equal to the magnitude of the difference between saidselected frequency and said pump frequency, a parallel resonant circuitcoupled to said diode and sharply tuned to said idler frequency forproviding a high impedance for signals -at said idler frequency and alow impedance for signals at said selected frequency, a series resonantcircuit coupled across said source means and tuned to said idlerfrequency, and an output circuit coupled to said diode and to saidparallel resonant circuit for translating amplified signals at saidselected frequency.

6. A parametric amplifier comprising: a variable cap-acitance diode,means for applying to said diode a direct bias and an alternating signalwhich varies at a pump frequency to provide in said diode a fixedcapacitance component and Ia capacitance component which varies at saidpump frequency, sourcey means for applying an input signal of afrequency higher than said pump frequency to said diode, inductancemeans coupled to said diode to provide series resonance with said fixedcap-acitance 4component at `an idler frequency equal to the differencebetween said input signal frequency and said pump frequency, a parallelresonant circuit coupled toV said diode and sharply tuned to said idlerfrequency for providing a high impedance for signals at said idlerfrequency and a low impedance for signals lat said input signalfrequency, a series resonant circuit coupled across said source meansand tuned to saidfidler frequency, and an output circuit coupled to saiddiode and to said parallel resonant circuit for translating amplifiedsignals at said input signal frequency.

7. A parametric amplifier comprising: `a variable capacitance diodehaving first and second electrodes, bias means having one `terminalcoupled to said first electrode for applying -a direct bias thereto,pump source means having a first terminal coupled to said secondelectrode and o having a second terminal coupled to another terminal ofsaid bias means for applying to said diode .an alternating signal whichvariesrat a pump frequency to provide in said diode a fixed capacitancecomponent and a capacitance `component which varies at said pumpfrequency,

.input source means coupled between said first and second terminals forapplying an input signal of a selected frequency to said diode, a firstcapacitor coupled between said Y-first and second terminals, a firstinductor coupled between said first electrode of said diode and said oneterminal of said bias means, said first inductor being ductor ,and athird capacitor coupled in series between said first Iand said secondterminals, said third inductor and said third capacitor being, resonantat said idler frequency.

l8. A parametric amplifier comprising: a variable capacitance diodehaving first and second electrodes, bias means having one terminalcoupled to said first electrode for applying a direct bias thereto, pumpsource means having a rst terminal coupled to said second electrode andhaving a Vsecond terminal coupled to another terminal of said bias meansfor applying to said diode an .alternating signal which varies at a pumpfrequency to provide in said diode a fixed capacitance component and acapacitance component which varies at said pump frequency, input sourcemeans coupled between said first and second terminals for applying aninput signal of .a frequency higher than said pump frequency to saiddiode, a first capacitor coupled between said first and secondterminals, a first inductor coupled between said first electrode of saiddiode and said one terminal of said bias means, said first inductorIbeing resonant with said fixed capacitance component at an idlerfrequency equal to the difference between said input'signal and saidpump frequencies, an output coupling impedance device having oneterminal coupled to said first electrode of said diode and havinganother terminal, a second inductor and a second capacitor coupled inparallel between said another terminal of said impedance device and saidone terminal of said bias means, said second inductor and said secondcapacitor being resonant at said idler frequency, and a third inductor.and a third capacitor coupled in series between said first and saidsecond terminals, said third inductor and said third capacitor beingresonant at said idler frequency.

References Citedrliy the Examiner Fisher: Proceedings of the 1nE,Ju1y1960, pp. 1227- 1232.

Varactor Applications, by Peneld et al., The M.I.T. Press, `Cambridge(Mass), 1962, pp. 1GO-165, 169-173.

ROY LAKE, Primary Examiner.

D. R. HOSTETTER, Assistant Examiner.

1. A PARAMETRIC AMPLIFIER COMPRISING: FIRST REACTANCE MEANS FORPROVIDING A FIXED REACTANCE COMPONENT AND A REACTANCE COMPONENT WHICH ISVARIABLE AT A PUMP FREQUENCY, MEANS FOR APPLYING AN INPUT SIGNAL TO ASELECTED FREQUENCY TO SAID FIRST REACTANCE MEANS, SECOND REACTANCE MEANSCOUPLED TO SAID FIRST REACTANCE MEANS TO PROVIDE SERIES RESONANCE WITHSAID FIXED REACTANCE COMPONENT AT AN IDLER FREQUENCY EQUAL TO THEMAGNITUDE OF THE DIFFERENCE BETWEEN SAID SELECTED FREQUENCY AND SAIDPUMP FREQUENCY, A PARALLEL RESONANT CIRCUIT COUPLED ACROSS SAID SECONDREACTANCE MEANS AND SHARPLY TURNED TO SAID IDLER FREQUENCY, AND A SERIESRESONANT CIRCUIT COUPLED ACROSS SAID INPUT SIGNAL APPLYING MEANS ANDTUNED TO SAID IDLER FREQUENCY.